Third Week of Development

During gastrulation, the embryo develops three germ layers (endoderm, mesoderm, and ectoderm) that differentiate into distinct tissues.

Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm.

Somites develop from the paraxial mesoderm and participate in the facilitation of multiple developmental processes.

The circulatory system develops initially via vasculogenesis, with the arterial and venous systems developing from distinct embryonic areas.

In the placenta, chorionic villi develop to maximize surface-area contact with the maternal blood for nutrient and gas exchange.